Components such as gas turbine blades, vanes and other cooled parts often contain cavities that distribute cooling air to a plurality of holes in the wall of the part which lead to the outer surface. Most turbine components are coated for protection from oxidation and/or corrosion with, for example, an MCrAIY coating (base coat) and some are also coated with a thermal barrier coating (TBC) for thermal insulation. The demands of operation of the parts in a gas turbine often lead to the degradation of the coating before the structural integrity of the underlying part itself is degraded. Hence, the base coat and TBC must be removed and re-applied.
The re-application of the coatings can be very problematic for parts with a large number of cooling holes. Often the base coat can reach thicknesses of 150-300 .mu.m, and the TBC may be another 200-500 .mu.m in thickness. The combined thicknesses of these coatings would have a very significant (and negative) influence on the effectiveness of the cooling holes if the coatings were deposited into these holes, especially considering that some holes are 1 mm or less in diameter. Specially shaped cooling holes are particularly susceptible to this as their effectiveness depends heavily on the accuracy of the shape of the hole.
There have been several disclosures relating to this problem and there are several widely known practices. Those skilled in the art are aware that a common practice is to braze or weld the holes closed with a suitable material after the old coatings have been removed, re-apply the new coatings, and re-manufacture the holes. The problem with this is that the brazing or welding operations introduce zones of weakness into the material. Normal hole manufacturing operations have errors associated with the placement of the holes, and when residual welding or brazing material is left, the zones of weakness go into operation with the part and compromise the mechanical integrity of the part.
One disclosure which offers a solution to this is U.S. Pat. No. 5,702,288, in which an abrasive slurry is injected into the cavity of the component and forced through the cooling holes which were partially covered by the coating material. There was no welding or brazing closed prior to coating. However this also abrades the internal cooling configuration (ribs), any inserts, and also the non-coated portion of the cooing holes.
Another disclosure which offers a better solution is U.S. Pat. No. 4,743,462, in which fugitive plugs are inserted into the cooling holes and partially volatilize during the coating process. The volatilization disrupts the coating in the region of the hole, and once the plugs are completely removed the holes are essentially free of coating and the cooling air will be unimpeded. The disadvantage of this method is that the large portion of the plug which blocks the surface does not constitute an even continuation of the cooling hole (it is specified to be larger than the cooling hole opening), therefore the path of the cooling air will be different from the intention of the design. This is particularly true for film cooling holes and specially shaped holes which are highly dependent on the shape of the hole near the external surface of the part. If the walls of the cooling holes are not maintained straight all the way through the coating layers (again, MCrAIY and TBC may amount to 0.8 mm or more in thickness) the cooling efficiency will be significantly compromised.
A further disadvantage of the method disclosed in U.S. Pat. No. 4,743,462 is that the plugs must all be placed individually into the cooling holes. For small simple aero-engine parts such as the one illustrated in the disclosure (containing only several rows of cooling holes at the leading edge) this is feasible, however for large turbine components of land-based gas turbines which may contain several hundred cooling holes, it is no longer feasible to individually place plugs into each hole. This is further complicated by the fact that each component may be manufactured with several different types of cooling holes--including conical, straight cylindrical and holes with changing wall angles. Each type of cooling hole would require its own specially designed plug.
A further disclosure in which all holes are plugged at once is given in U.S. Pat. No. 5,800,695. A masking agent is placed into the cooling configuration and forced through until it fills the cooling holes from the inside, but only up to the level of the exterior surface of the component. A coating is then applied, in this case electrolytically applied platinum. Due to the non-conductivity of the plastic maskant cited in the disclosure, no Pt would be deposited on the masking agent in the cooling holes. However, if the coating were deposited using thermal spraying techniques, it would coat over the maskant in the cooling holes, forming a layer that would remain after maskant removal. This layer would have to be removed. No solution is offered for this problem--in particular, how to remove the layer of coating material in such a way that the intended cooling hole design is maintained through the thickness of the coating material.
In addition, only plastic materials are specified as maskant materials, and in U.S. Pat. No. 4,743,462 the mask material is specified to volatilize at a temperature below that of the deposition process. The problem with this is that part requiring a MCrAIY coating and TBC must have the MCrAIY coating "diffusion bonded" by a high temperature heat treatment (about 1000.degree. C.-1150.degree. C. in vacuum) before the TBC can be applied. These specified materials would not be retained for the TBC coating process, and would either have to be re-applied, or the advantage of the masking would be lost. Indeed, in patent U.S. Pat. No. 5,800,695 the example process clearly states that after electrolytic platinum coating, the maskant is removed and then the parts are aluminized, with no mention of protecting the cooling holes from Al deposition.